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7. If the statement is a UNION of SELECT statements, merge the query results for the individual statements into a single table of query results. Eliminate duplicate rows unless UNION ALL is specified. 8. If there is an ORDER BY clause, sort the query results as specified. The rows generated by this procedure comprise the query results. Following the previous steps: 1. The FROM clause generates all possible combinations of rows from the CUSTOMERS table (21 rows) and the ORDERS table (30 rows), producing a product table of 630 rows. 2. The ON clause selects only those rows of the product table where the customer numbers match (CUST_NUM = CUST), reducing the 630 rows to only 30 (one for each order). 3. The WHERE clause selects only those rows of the resulting table where the customer number is the one specified (CUST_NUM = 2103). Only four rows are selected; the other 26 rows are eliminated. 4. The SELECT clause extracts the three requested columns (COMPANY, ORDER_ NUM, and AMOUNT) from each remaining row of the resulting table to generate four rows of detailed query results. 5. The ORDER BY clause sorts the four rows on the ORDER_NUM column to generate the final query results. Obviously no SQL-based DBMS would actually carry out the query this way, but the purpose of the previous definition is not to describe how the query is carried out by a DBMS. Instead, it constitutes a definition of how to figure out exactly what a particular multitable query means that is, the set of query results that it should produce.
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The SQL join operation combines information from two tables by forming pairs of related rows from the two tables where the matching columns in each of the tables have the same values. If one of the rows of a table is unmatched in this process, the join can produce unexpected results, as illustrated by these queries: List the salespeople and the offices where they work.
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SELECT NAME, REP_OFFICE FROM SALESREPS; NAME REP_OFFICE -------------- ----------Bill Adams 13 Mary Jones 11 Sue Smith 21 Sam Clark 11 Bob Smith 12 Dan Roberts 12 Tom Snyder NULL
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Recall that not all SQL tools display NULL values in the manner shown. List the salespeople and the cities where they work.
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SELECT NAME, CITY FROM SALESREPS JOIN OFFICES ON REP_OFFICE = OFFICE; NAME -------------Mary Jones Sam Clark Bob Smith Paul Cruz Dan Roberts Bill Adams Sue Smith Larry Fitch Nancy Angelli CITY -----------New York New York Chicago Chicago Chicago Atlanta Los Angeles Los Angeles Denver
PART II
Based on the English-language descriptions of these two queries, you would probably expect them to produce ten rows, one for each salesperson. The first query indeed produces ten rows, but the second query produces only nine rows. Why Because Tom Snyder is currently not assigned to an office. His row has a NULL value in the REP_OFFICE column (which is the matching column for the join). This NULL value doesn t match any of the office numbers in the OFFICES table, so Tom s row in the SALESREPS table is unmatched. As a result, it vanishes in the join, whether the join is specified using the ON clause or the WHERE clause. The standard SQL join thus has the potential to lose information if the tables being joined contain unmatched rows. Based on the English-language version of the request, you would probably expect the second query to produce results like these: List the salespeople and the cities where they work.
SELECT NAME, CITY FROM SALESREPS LEFT OUTER JOIN OFFICES ON REP_OFFICE = OFFICE; NAME -------------Tom Snyder Mary Jones Sam Clark Bob Smith Paul Cruz Dan Roberts Bill Adams Sue Smith Larry Fitch Nancy Angelli CITY -----------NULL New York New York Chicago Chicago Chicago Atlanta Los Angeles Los Angeles Denver
Part II:
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